In a distribution transformer having liquid dielectric, in the event of a fault there is a risk of the dielectric overheating, thereby giving rise to excessive pressure that can reach a value so high that the transformer explodes. There is then a risk that the dielectric might be sprayed around, with serious consequences on the environment.
French Patent Application FR2801141 describes a protection system making it possible to prevent the transformer from exploding. In that protection system for protecting a three-phase distribution transformer immersed in a liquid dielectric contained in a tank, two of the three phases are equipped on the high-voltage side of the transformer with respective current-limiting fuses, and with respective protection micro-fuses disposed in series with the corresponding current-limiting fuse, each of said micro-fuses detecting abnormal current conditions and being associated with a striker.
That system further comprises at least one fault detector for detecting faults on the basis of at least one of the following indicators: pressure in the tank, and level of the dielectric. It further comprises a short-circuiter situated on the high voltage side between the current-limiting fuses and the high-voltage windings of the transformer, said short-circuiter being capable of being triggered directly by the fault detector, or else by the striker associated for example with each micro-fuse, and in the event of the micro-fuse operating. On the high-voltage side of the transformer and upstream from the short-circuiter, the third phase is equipped with a micro-switch associated with a striker triggering the short-circuiter in the event of the micro-switch operating.
The micro-switch is equivalent to a micro-fuse provided with an isolating function, and said micro-switch has a calibrated conductive blade for triggering the interruption of fault currents between the third phase and the tank of the grounded transformer.
However, in the event of a series of relatively exceptional circumstances, it is possible that a grounded residual fault current which is greater than the interruption threshold of the micro-switch subsists in the third phase. Under such circumstances, the transformer windings are no longer isolated.